Two oligomeric forms of plasma ficolin have differential lectin activity

Ficolins are plasma proteins with binding activity for carbohydrates, elastin, and corticosteroids. The ficolin polypeptide has a collagen-like domain that presumably brings three subunits together in a triple helical rod, a C-terminal fibrinogen-like domain (fbg) similar to that of tenascin, which presumably has the binding activities, and a small N-terminal domain that we find to be the primary site for forming the ficolin oligomer. By sedimentation equilibrium we determined that the main plasma form, which we call big ficolin, had mass of 827,000 Da, consistent with 24 subunits. Little ficolin, about half this size, was obtained after binding to a GlcNAc affinity column. Electron microscopy of little ficolin showed a parachute-like structure, with a small globe at one end, corresponding to the 12 N-terminal domains, and the fbg domains clustered together at the ends of the collagen rods. Big ficolin was formed by the face to face fusion of the fbg domains of two little ficolins, leaving the rods and N-terminal domains projecting at opposite ends. Little ficolin maintained a high affinity for the GlcNAc column, and big ficolin had a low affinity or none. The binding sites for ligands may be obscured in this big ficolin oligomer, providing a regulation of their activity.